Upgrade Spymemcached to 2.10.0.

This changeset upgrades the spy dependency and also applies all
needed fixes for the upgrade (it passes the executor along to the
futures).

Change-Id: Icea367bbe66aa9a3eaea9c85c307e04bf353840f
Reviewed-on: http://review.couchbase.org/28815
Reviewed-by: Michael Nitschinger <michael.nitschinger@couchbase.com>
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>